Original question:
Hi team,
What is the open-loop output resistance of this device? I cannot find this value in the datasheet.
I have also one common question here, how to calculate the closed-loop output impedance, such as using OPA4348 as a buffer? Thanks.
This OPA348 is a fairly old model,
THe PDS shows nothing as you say, the 2011 model does not list Zol as a modeled parameter, but there is something (likely not accurate) in the model.
Here is a Zol sim, looks like 1kohm with a single pole,
Hello Charles,
I've asked internally and this information does not seem to be available anywhere. Most likely, it was not characterized when the part was released. Unfortunately, this is an older part and the data collected for the parts has changed over time. Is there any way to convince the customer to move to another part? I think there would might be a lot of advantages to the TLV9004 over the OPA4348.
As for your second question, when you put the amplifier into a negative feedback configuration, the output impedance of the amplifier drops drastically. You can find this analysis online, but the key equation is the following:
Here, let R_out be the closed loop output impedance, R_O be the open loop output impedance, A_OL be the open loop output impedance, and Beta be the feedback factor. Let's say you are using the TLV9004, which has an almost constant output impedance of 1200Ohms across frequency, in a buffer configuration (Beta = 1). Assuming a conservative open loop gain of 100dB, or 100,000 V/V, your effective closed-loop output resistance for the amplifier is only 12mOhms! So the output impedance of the amplifier approaches that of an ideal amplifier.
Note that Beta will decrease as you increase your closed-loop gain. So for larger closed loop gains, the closed-loop output impedance will increase.
Let me know if you need any further information.
Regards,
Daniel
